1. Field of the Invention
The present invention relates to a metal ring of a metal belt used in a belt type continuously variable transmission, wherein a free state radius of the metal ring, when cut, is set to increase the fatigue life of the metal ring.
2. Description of the Related Art
A metal belt used in a belt type continuously variable transmission includes a metal ring assembly formed by layering a plurality of metal rings on top of each other, and a plurality of metal elements supported along the metal ring assembly. Tensile stress and compressive stress due to flexure, as well as tensile stress due to tension, act on each metal ring. Specifically, in a wound section wherein the metal ring is wound around a pulley, an outer peripheral face of the metal ring is pulled, and a tensile stress acts thereon. At the same time, an inner peripheral face of the metal ring is compressed, and a compressive stress acts thereon. In a chord section where the metal ring is separated from the pulley, the outer peripheral face of the metal ring is compressed, and a compressive stress acts thereon. At the same time, the inner peripheral face thereof is pulled, and a tensile stress acts thereon. In this way, the stress acting on the inner and the outer peripheral faces periodically varies with the rotation of the metal ring such that the fatigue life of the metal ring deteriorates.
Japanese Patent Application Laid-open No. 2003-126933 discloses an arrangement in which a free state radius Ro of the metal ring is set so that, in a TOP ratio state in which the metal ring receives the most severe load, a corrected stress amplitude σao′ (a stress amplitude σao when a stress center σmo=0) on the outer peripheral face of the metal ring, calculated from the stress amplitude σao and the stress center σmo on the outer peripheral face, coincides with a corrected stress amplitude σai′ (a stress amplitude σai when a stress center σmi=0) on the inner peripheral face of the metal ring calculated from the stress amplitude σai and the stress center σmi of the inner peripheral face.
The inner peripheral face of the metal ring, that is, the innermost layer of the plurality of stacked metal rings, abuts a saddle face of a metal element, which is a transverse element of the metal belt. Accordingly, the maximum compressive stress on the inner peripheral face of the metal ring increases by an amount corresponding to a compressive stress (contact stress) acting on the abutment section. Therefore, during operation of the belt type continuously variable transmission, if the sum of a residual compressive stress of the metal ring due to a nitriding treatment, a compressive stress due to flexure, and a contact stress due to contact of the metal element with the saddle face becomes excessive and exceeds an elastic deformation limit, the metal ring plastically deforms and becomes vulnerable to breaking. The arrangement disclosed in Japanese Patent Application Laid-open No. 2003-126933 does not take into account either the contact stress due to contact with the saddle face acting on the inner peripheral face of the metal ring of the innermost layer, or the relationship between the maximum compressive stress and the elastic deformation limit of the metal ring, leading to a possibility that the life of the metal ring might not be increased as desired.